Fix mocking in TestPluginBasedTask unit tests
Fix mocking in TestPluginBasedTask unit tests

Return a real workflow instance like the other test functions in this
test module.

Signed-off-by: Adam Cmiel <1217f9865bd733d1bcad0d0d64be310272c5592c@redhat.com>

projectatomic/atomic-reactor,fr34k8/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor